John Grant, I have a similar experience. I have tried two Android devices. A phone; Samsung Note 4 running Android 6.0.1. And a tablet; Lenovo Tab 2 A8-50F running Android 5.1. they were connected using a USB OTG cable.
It would not surprise me if the Lenovo Tablet didn't support MIDI. But, the Note 4 specifically had a SDK put out by Samsung, to enable MIDI interfacing.
I was attempting to connect to an electronic piano, a Williams Overture 2, so that I could record the MIDI performance of the pianist.
I tried 4 different Android apps on the Lenovo Tab, and 3 others on my Note 4.
I am even open to buying a new tablet, but I somehow need to figure out if it works before I buy.